diff --git a/src/protocol/ast.rs b/src/protocol/ast.rs
index a8b18a801eef40f745879bd48630af59c8159dc3..7657a9390ebffa00927a9cdc1bc6ae71225e0df2 100644
--- a/src/protocol/ast.rs
+++ b/src/protocol/ast.rs
@@ -1357,7 +1357,7 @@ pub struct SelectStatement {
 
 #[derive(Debug, Clone)]
 pub struct SelectCase {
-    pub guard_var: Option<MemoryStatementId>, // optional memory declaration
+    pub guard_var: MemoryStatementId, // invalid ID if there is no declaration of a variable
     pub guard_expr: ExpressionStatementId, // if `guard_var.is_some()`, then always assignment expression
     pub block: BlockStatementId,
 }